The present invention relates to a syringe for injecting an extemporaneous mixture, the syringe being of the type comprising:                a body containing at least two substances initially separated by an intermediate piston, which body has two relatively movable portions that are movable between an initial position in which the two substances are separate and a final position in which the two substances are mixed together to form an extemporaneous mixture;        automatic means for injecting the extemporaneous mixture out from the body in order to proceed with injection; and        latch means for preventing triggering of the automatic injection means.        
Such a syringe is described in particular in document EP-A-0 219 899 with reference to FIGS. 14 and 15.
In that document, the syringe includes an injection cartridge received in a syringe casing. The cartridge constitutes a tubular tank containing the substances to be injected. These substances are initially separated by moving pistons. The means for mixing the substances are formed by a lateral projection of the cartridge. This locally increases the internal section of the cartridge and defines a passage enabling the substances to go past the separator pistons.
Means for automatically actuating the rear wall in order to inject the extemporaneous mixture are also provided in the casing. They essentially comprise a spring-loaded pusher together with means for releasing the pusher. In order to avoid accidental release of the means for injecting the extemporaneous mixture, a removable member is provided for latching the automatic injection means. That latching member has a pin which is initially engaged between resilient tongues that retain the spring. The pin prevents the spring being released by preventing the tongues from deforming.
In order to reconstitute the extemporaneous mixture, at least a portion of the syringe casing can be moved relative to the cartridge so that when said portion of the casing is caused manually to slide over the cartridge, the pusher urges the moving rear partition of the cartridge so as to move it towards the front partition, thereby causing the substances for injection to be mixed together by passing through the lateral projection.
After the substances for injection have been mixed together, and after the latching member has been withdrawn, the spring of the actuator means is released, thereby causing the previously mixed-together substances to be injected automatically.
In practice, it is found that the operator sometimes releases the spring of the actuator means to proceed with injection before the substances have been fully mixed together, with the moving portion of the casing not having been caused to slide along its entire stroke along the cartridge. Extemporaneous mixing is then either not performed or is performed incompletely only, such that it has the wrong concentrations of the various ingredients initially contained in the cartridge.
The effect of this faulty operation is particularly unfortunate in that self-injection syringes are commonly used in emergency situations where people who are on their own need to inject themselves. Stress can then cause the user to act hastily without ensuring that the mixture has been properly and completely reconstituted.